The commercial material of the alignment layer of the LCD is based on a so-called polyimide (PI), however the conventional polyamide is a material that is nearly insoluble, unmelting, thermal-resistant and chemical-resistant. It is very inconvenient for this polyimide to be processed, and in particular applied in making the alignment layer of the LCD.
Conventional aromatic polyimides have good thermal resistance, and these polyimides are usually obtained from the condensation products of aromatic anhydrides or derivatives and aromatic diamines. First, the polyamidic acid is synthesized. Then, a crosslinked imidization is performed under a high temperature to produce the desired polyimide.
The commercial kit of the alignment layer is usually sold in the form of solution containing the intermediates of the reaction used to synthesize the desired polyamide. First, this solution is coated on a substrate, then the substrate is exposed to a high temperature to allow the polyamidic acid cyclize partially or entirely. The polyamidic acid is not a stable substance, so it will easily decompose into low M.W. substances. Therefore, the product composed of this polyimide is not stable. Accordingly, the usages of the conventional polyimides are limited by their preservation (short retention time and low temperature preservation) and cost.
In addition, the process needs a high-temperature long-term baking; therefore, the conductive layer on the substrate is easily oxidized, resulting in the increase at the operating voltage and power consumption. Moreover, the high-temperature baking makes it hard to fabricate the color filter.
A method for making a polyimide synthesized through the intermediate polyamidic acid polymerized by aliphatic anhydrides and diamines was disclosed by Hitachi Chem. Co. Ltd. (Japan unexamined Patent No. 60-135430). However, this method can not provide useful solvent soluble polyimides.
Similarly, a method for making a solvent soluble polyimide by selecting suitable monomers and assembly thereof was disclosed by Hitachi Chem. Co. Ltd. (Japan unexamined Patent No. 59-179623). Unfortunately, the polyimide film appears dark, which is adverse to optical transparency. Accordingly, the polyamides made according to this method can not be used as the material of the alignment layer of the LCD.
In accordance, it is necessary to develop a solvent soluble polyimide, which can be well preserved and used, to make a product with good electric conductivity and stability, and in particular the material of the alignment layer of the LCD.